See if any of the following situations match yours:
- Your pay as you go was turned off initially. After finishing your data pack, you enabled pay as you go.
- You have used all of your pay as you go data and then you bought a data pack
- You have increased your pay as you go limit than the original limit while using pay as you go.
- You have decreased your pay as you go limit than original limit while using pay as you go.
If the above cases occur, then you simply have to turn your phone data off, take a breath and turn it on again. Your internet should be running at full speed now, go check!